A bipolar power amplifier (using, e.g., a heterojunction bipolar transistors (HBT)) has a long drift base layer in a tradeoff of collector-emitter drive current for base control. A high Ge gradient in the base layer is concurrently used to provide drift fields for high-frequency operation. A total Ge dose (e.g., Ge concentration percentage over thickness), though, is well under a critical limit so that relaxation does not occur in the base layer. When relaxation occurs, ensuing dislocations generally thread through the entire base layer to a surface of a lower device layer (e.g., a collector layer), and the dislocations are electrically active. These dislocations form recombination centers for minority carriers, or provide short circuit paths between the collector layer and an emitter layer of the HBT. These effects increase a base current as well as a collector-emitter leakage, in the HBT.
Currently, the HBT is manufactured with only a single straining material including a compressive SiGe material in the base layer. Carbon incorporated in the base layer serves the purpose of only suppressing dopant diffusion, particularly, transient enhanced diffusion (TED). Accordingly, an amount of the carbon is low, e.g., 3E19 cm−3. However, under such a design, relaxation and ensuing dislocations in the HBT still can occur. In addition, the current HBT may not be useable for power amplifiers requiring higher current gain and higher-frequency operation.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.